Electronics > Beginners

Measuring Mains with Rigol DS1052E

<< < (4/8) > >>

Kiriakos-GR:

--- Quote from: ciccio on August 02, 2011, 06:13:32 pm ---Are you sure that neutral on your socket is at ground potential?
Measure it, please.
Here in Italy neutral is grounded only at the electric company's medium voltage to low voltage transformer,
The one serving my home is about 500 meter from me, and in this moment  neutral is about 30 V respect to ground...
An old electrician told me: live is hot, and neutral is hot..

--- End quote ---

Yes and No.

The neutral can show voltage , when and only if there is extreme loads on the phase.
When the load drops, the neutral voltage gets lower too.
The voltage on the neutral commonly called as " returning currents ".
It is part of the energy that phase has, that it does not consumed by the electrical device,
and so one part of it returns on the neutral wire. 

The major problem that we have in the industry is that the loads are so large ones, and the neutral gets a significant amount of voltage.
You have no choice other than to ground it  when ever is possible.
But if you do so, there is no way to install protection relays afterward s. 

alm:

--- Quote from: IanB on August 02, 2011, 06:53:45 pm ---Either way, if there is 30 V between neutral and earth, it suggests there is some significant voltage drop where it ought not to be; for instance there might be an unexpected current flowing in the earth conductor?

--- End quote ---
How about current flowing through the neutral conductor? V=IR, so if there is a lot of current and some resistance, there will be a voltage drop. 30V sounds like a lot though.


--- Quote from: IanB on August 02, 2011, 06:53:45 pm ---I suppose though that I am wrong to think you could connect earth and neutral inside a device; most likely you should not do so and I imagine I would find that out if I could locate the right regulations to read...

--- End quote ---
The only point were they may be connected is at the distribution level (may be in your own home, may be somewhere far away depending on the topology). Connecting them somewhere else may result in fireworks and will likely trigger the GFCI. They may also not be connected at all, the ground may also be in the middle between neutral and ground. A break in the neutral wire somewhere may also change the voltage on the neutral wire, since the equipment connected to the phases now forms a voltage divider, so a large unbalance between the phases will cause neutral to be far away from ground.

Equipment with the chassis connected to neutral has not been allowed for a long time, for good reasons. I'm not sure about the UK, but in many countries live and neutral are often wired incorrectly. Reversing ground and neutral is also somewhat common, but that gets noticed fairly quickly because it will trip the GFCI. Reversing live and neutral can remain undetected for a long time since most equipment developed for an international market doesn't care. Most safety regulations I'm aware of treat live and neutral exactly the same, both require the same clearance, creapage and isolation.

If you're not familiar with this, I think building something connecting directly to mains is a dangerous plan.

Alex:

--- Quote from: Kiriakos-GR on August 02, 2011, 02:38:05 pm ---I think that logicnibble had set one interesting question.

And no it does not limited to the Rigol product range.

As a electrician I would like to have on screen other than volts and the wave form too.
So to be able to see and tell how perfect or clean is the AC wave at 220 or even at 380V.
Or even, to be able to check DC/AC inverters and see how they respond against a specific load.

Some people they do complains that the low quality DC/AC inverters cause noisy operation on small motors, usually small pumps.
I like to be able to see of what is happening with the wave forms.

If there is a tool capable to help me out there, I need to know which one is capable for the task.


 

--- End quote ---

Crest factor (http://en.wikipedia.org/wiki/Crest_factor) might be a useful indicator for what you are describing. Measurement of crest factor usually comes as a measurement in DMMs, oscilloscope and scopemeters. Of course, having a proper display of the waveform gives more insight, but a DMM with crest factor measurment should be able to tell you if you need to get the scope out. Just a thought.

IanB:

--- Quote from: alm on August 02, 2011, 11:01:33 pm ---How about current flowing through the neutral conductor? V=IR, so if there is a lot of current and some resistance, there will be a voltage drop. 30V sounds like a lot though.

--- End quote ---
I thought it seemed like a lot. A 30 V drop on a main conductor would seem to me something like a brown out.


--- Quote from: alm on August 02, 2011, 11:01:33 pm ---If you're not familiar with this, I think building something connecting directly to mains is a dangerous plan.

--- End quote ---
Fair enough. I don't do electricity for my day job and I also don't like to just follow rules by rote without understanding them. But once I followed through the reasoning as you explained above I had no problem seeing the picture. My incorrect thinking was not something that was going to follow through into a real project without me conducting a proper investigation. Any exposed metal parts should be earthed and current carrying conductors should be properly insulated and should be connected only to the parts of the circuit that are meant to be energized.

PetrosA:
There have been some good (and some not-so-good...) opinions on the 30V to ground measurement. In most modern systems where the neutral is grounded at or near the main disconnect or last transformer feeding a service, 30V could likely indicate one of a number of possible serious problems.

1 - there is a bad (high impedance) neutral connection between your outlet and the neutral-earth bond
2 - you are connected to one of three phases (typical in EU) and there is a large imbalance between the phases
3 - you are connected to one of three phases and there are a lot of non-linear loads on the transformer causing an overload on the neutral (this is actually more likely now that the EU is forcing the use of compact fluorescent bulbs).
4 - the neutral from the transformer is undersized and overheating
5 - the ground is actually disconnected and you are measuring induced voltage on the ground wire to the properly grounded neutral

There are other possibilities, but these would be the main culprits.

If your service is protected by an RCD then you should be relatively safe. I would still have it checked by an electrician though since any of the neutral problems could potentially create a fire or shock hazard.

If there are large voltages like this between ground and neutral, please be VERY careful testing it because there's the possibility that connecting the two will shunt a large load (10s of amps) through whatever is jumpering them.

Navigation

[0] Message Index

[#] Next page

[*] Previous page

There was an error while thanking
Thanking...
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod